1. Field of the Invention
The present invention relates to a method for providing hearing assistance to a user; it also relates to a corresponding system. In particular, the invention relates to a system comprising a microphone arrangement for capturing audio signals, audio signal processing means and means for stimulating the hearing of the user according to the processed audio signals.
2. Description of Related Art
One type of hearing assistance systems is represented by wireless systems, wherein the microphone arrangement is part of a transmission unit for transmitting the audio signals via a wireless audio link to a receiver unit comprising or being connected to the stimulating means. Usually in such systems the wireless audio link is an narrow band FM radio link. The benefit of such systems is that sound captured by a remote microphone at the transmission unit can be presented at a much better SNR to user wearing the receiver unit at his ear(s).
According to one typical application of such wireless audio systems, the stimulating means is loudspeaker which is part of the receiver unit or is connected thereto. Such systems are particularly helpful in teaching environments for normal-hearing children suffering from auditory processing disorders (APD), wherein the teacher's voice is captured by the microphone of the transmission unit, and the corresponding audio signals are transmitted to and are reproduced by the receiver unit worn by the child, so that the teacher's voice can be heard by the child at an enhanced level, in particular with respect to the background noise level prevailing in the classroom. It is well known that presentation of the teacher's voice at such enhanced level supports the child in listening to the teacher.
According to another typical application of wireless audio systems the receiver unit is connected to or integrated into a hearing instrument, such as a hearing aid. The benefit of such systems is that the microphone of the hearing instrument can be supplemented or replaced by the remote microphone which produces audio signals which are transmitted wirelessly to the FM receiver and thus to the hearing instrument. In particular, FM systems have been standard equipment for children with hearing loss in educational settings for many years. Their merit lies in the fact that a microphone placed a few inches from the mouth of a person speaking receives speech at a much higher level than one placed several feet away. This increase in speech level corresponds to an increase in signal-to-noise ratio (SNR) due to the direct wireless connection to the listener's amplification system. The resulting improvements of signal level and SNR in the listener's ear are recognized as the primary benefits of FM radio systems, as hearing-impaired individuals are at a significant disadvantage when processing signals with a poor acoustical SNR.
Most FM systems in use today provide two or three different operating modes. The choices are to get the sound from: (1) the hearing instrument microphone alone, (2) the FM microphone alone, or (3) a combination of FM and hearing instrument microphones together.
Usually, most of the time the FM system is used in mode (3), i.e. the FM plus hearing instrument combination (often labeled “FM+M” or “FM+ENV” mode). This operating mode allows the listener to perceive the speaker's voice from the remote microphone with a good SNR while the integrated hearing instrument microphone allows to listener to also hear environmental sounds. This allows the user/listener to hear and monitor his own voice, as well as voices of other people or environmental noise, as long as the loudness balance between the FM signal and the signal coming from the hearing instrument microphone is properly adjusted. The so-called “FM advantage” measures the relative loudness of signals when both the FM signal and the hearing instrument microphone are active at the same time. As defined by the ASHA (American Speech-Language-Hearing Association 2002), FM advantage compares the levels of the FM signal and the local microphone signal when the speaker and the user of an FM system are spaced by a distance of two meters. In this example, the voice of the speaker will travel 30 cm to the input of the FM microphone at a level of approximately 80 dB-SPL, whereas only about 65 dB-SPL will remain of this original signal after traveling the 2 m distance to the microphone in the hearing instrument. The ASHA guidelines recommend that the FM signal should have a level 10 dB higher than the level of the hearing instrument's microphone signal at the output of the user's hearing instrument.
When following the ASHA guidelines (or any similar recommendation), the relative gain, i.e. the ratio of the gain applied to the audio signals produced by the FM microphone and the gain applied to the audio signals produced by the hearing instrument microphone, has to be set to a fixed value in order to achieve e.g. the recommended FM advantage of 10 dB under the above-mentioned specific conditions. Accordingly, heretofore—depending on the type of hearing instrument used—the audio output of the FM receiver has been adjusted in such a way that the desired FM advantage is either fixed or programmable by a professional, so that during use of the system the FM advantage—and hence the gain ratio—is constant in the FM+M mode of the FM receiver.
EP 0 563 194 B1 relates to a hearing system comprising a remote microphone/transmitter unit, a receiver unit worn at the user's body and a hearing aid. There is a radio link between the remote unit and the receiver unit, and there is an inductive link between the receiver unit and the hearing aid. The remote unit and the receiver unit each comprise a microphone, with the audio signals of theses two microphones being mixed in a mixer. A variable threshold noise-gate or voice-operated circuit may be interposed between the microphone of the receiver unit and the mixer, which circuit is primarily to be used if the remote unit is in a line-input mode, i.e. the microphone of the receiver then is not used.
WO 97/21325 A1 relates to a hearing system comprising a remote unit with a microphone and an FM transmitter and an FM receiver connected to a hearing aid equipped with a microphone. The hearing aid can be operated in three modes, i.e. “hearing aid only”, “FM only” or “FM+M”. In the FM+M mode the maximum loudness of the hearing aid microphone audio signal is reduced by a fixed value between 1 and 10 dB below the maximum loudness of the FM microphone audio signal, for example by 4 dB. Both the FM microphone and the hearing aid microphone may be provided with an automatic gain control (AGC) unit.
WO 2004/100607 A1 relates to a hearing system comprising a remote microphone, an FM transmitter and left- and right-ear hearing aids, each connected with an FM receiver. Each hearing aid is equipped with a microphone, with the audio signals from a remote microphone and the respective hearing aid microphone being mixed in the hearing aid. One of the hearing aids may be provided with a digital signal processor which is capable of analyzing and detecting the presence of speech and noise in the input audio signal from the FM receiver and which activates a controlled inverter if the detected noise level exceeds a predetermined limit when compared to the detected level, so that in one of the two hearing aids the audio signal from the remote microphone is phase-inverted in order to improve the SNR.
WO 02/30153 A1 relates to a hearing system comprising an FM receiver connected to a digital hearing aid, with the FM receiver comprising a digital output interface in order to increase the flexibility in signal treatment compared to the usual audio input parallel to the hearing aid microphone, whereby the signal level can easily be individually adjusted to fit the microphone input and, if needed, different frequency characteristics can be applied. However, is not mentioned how such input adjustment can be done.
Usually FM or inductive receivers are equipped with a squelch function by which the audio signal in the receiver is muted if the level of the demodulated audio signal is too low in order to avoid user's perception of excessive noise due a too low sound pressure level at the remote microphone or due to a large distance between the transmission unit and the receiver unit exceeding the reach of the FM link, see for example EP 0 671 818 B1 and EP 1 619 926 A1. Contemporary digital hearing aids are capable of permanently performing a classification of the present auditory scene captured by the hearing aid microphones in order to select that hearing aid operation mode which is most appropriate for the determined present auditory scene. Examples of such hearing aids including auditory scene analysis can be found in US 2002/0037087, US 2002/0090098, WO 02/032208 and US 2002/0150264.
Further, binaural hearing systems are available, wherein there is provided a usually wireless link between the right ear hearing aid and the left ear hearing aid for exchanging data and audio signals between the hearing aids for improving binaural perception of sound. Examples of such binaural systems can be found in EP 1 651 005 A2, US 2004/0037442 A1 and U.S. Pat. No. 6,549,633 B1. In EP 1 531 650 A2 a binaural system is described wherein in addition to the binaural link a wireless audio link to a remote microphone is provided. A similar system is described in WO 02/074011 A2.
Hearing aids comprising an acoustic beam-former are described, for example, in EP 1 005 783 B1, EP 1 269 576 B1, EP 1 391 138 B1, EP 1 303 166 A2 and WO 00/68703.
According to EP 1 303 166 A2 and WO 00/68703, the direction of the formed acoustic beam is controlled by the measured direction of arrival (DOA) of the sound captured by the microphones. The DOA can be estimated by comparing the audio signals captured by a plurality of spaced apart microphones, for example, by comparing the respective phases. If the microphones are directional microphones, the DOA may be calculated by forming level ratios of the audio signals, see, for example, WO 00/68703. With two microphones the DOA can be estimated in two dimensions, and with three microphones the DOA can be estimated in three dimensions.
According to EP 1 303 166 A2 the audio signal processing is switched from an omni-directional mode to a directional mode once the voice of a certain speaker has been recognized by identifying the speaker from a plurality of known speakers. The DOA of the voice of the speaker is estimated and the result is used to set the beam former such that it points into this direction.
EP 1 320 281 A2 relates to a binaural hearing system comprising a beam former, which is controlled by the DOA determined separately for each of the left ear unit and the right ear unit, which each are provided with two spaced-apart microphones.
EP 1 691 574 A2 relates to a wireless system, wherein the transmission unit comprises two spaced-apart microphones, a beam former and a classification unit for controlling the gain applied in the receiver unit to the transmitted audio signals according to the presently prevailing auditory scene. The classification unit generates control commands which are transmitted to the receiver unit via a common link together with the audio signals. The receiver unit may be part of or connected to a hearing instrument. The classification unit comprises a voice energy estimator and a surrounding noise level estimator in order to decide whether there is a voice close to the microphones or not, with the gain to be applied in the receiver unit being set accordingly. The voice energy estimator uses the output signal of the beam former for determining the total energy contained in the voice spectrum.
It is an object of the invention to provide for a hearing assistance system and method which allows for particularly reliable detection of the presence of a voice source close to the microphone arrangement.